Apparatus and method of balancing a shaft

ABSTRACT

An apparatus for balancing a shaft of an aircraft engine preferably includes a plate and the use of standard fasteners. The plate provides mounting for a nose cone, and the balancing apparatus may be accessed from the front of the assembled engine.

FIELD OF THE INVENTION

The present invention generally relates to a balancing apparatus andmethod, and more particularly to an apparatus and method for balancing ashaft of an aircraft turbine engine.

BACKGROUND OF THE INVENTION

The main drive shafts of aircraft gas turbine engines are subject tomass imbalance due to manufacturing variations. The shafts must bebalanced prior to assembly in the engine in order to reduce shaftvibration during engine operation.

Shaft imbalance is characterized by a magnitude of imbalance and anangular direction of imbalance. The magnitude of imbalance caused by aneccentric rotating mass, is a function of the weight of the mass and theradial distance of the mass from the axis of rotation. The angulardirection of imbalance is determined by the angular position of theeccentric mass relative to an arbitrary reference direction.

Due to the subsequent operations of an aircraft turbine engine assembly,there is often a need for further balancing of the rotating parts whenthe engines are completed. Further balance adjustment however, isnormally difficult due to the inaccessibility of the rotating parts and,quite frequently, the unbalance is so severe that it is necessary tostrip the turbine engines for rebalancing.

Therefore, there is a need for an improved balancing apparatus for theshafts of aircraft turbine engines which will enable a shaft to bebalanced accurately and simply and which will permit simplified finaladjustment after the aircraft engine has become completely assembled.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an apparatus and amethod for balancing a shaft of an aircraft engine which is simple andallows for simplified final adjustment after the aircraft engine hasbeen assembled.

In accordance with one aspect of the present invention, there is anapparatus provided for balancing a shaft of an aircraft engine whichcomprises a round plate and a plurality of standard mechanicalfasteners. The round plate defines a first group of holes axiallyextending therethrough. The round plate is coaxially attached to theshaft at a forward end thereof. The standard mechanical fasteners arefor selectively engaging in at least one of the holes of the first groupwhich is determined in a shaft balancing test, thereby adding a selectedbalance weight to the plate in the determined hole, in order to balancethe shaft.

The plate preferably also comprises a second group of holes axiallyextending therethrough for receiving mounting bolts therein for mountinga nose cone to the aircraft engine.

In accordance with another aspect of the present invention, there is anapparatus provided for an aircraft engine which comprises a nose cone ofthe aircraft engine, at least one balance weight element, and a memberadapted to be centrally mounted to a forward end of a rotatable shaft ofthe aircraft engine. The member includes a plurality of mounting pointsby which the nose cone is mounted to the member, and further includes aplurality of attachment points to cooperate with the at least onebalance weight element to retain the same to the member. The at leastone weight element is retained in a selected one of the attachmentpoints such that the shaft is rotationally balanced.

In accordance with a further aspect of the present invention, there is amethod provided for balancing a shaft of an aircraft engine in which theshaft includes a mounting plate for mounting a nose cone to one sidethereof. The method comprises steps of, with the nose cone dismounted,observing a rotational imbalance of the shaft; and accessing themounting plate through a front opening of a casing of the aircraftengine to install and affix at least one standard fastener in one of aplurality of axial holes of the mounting plate determined during theobservation step, thereby rotationally balancing the shaft.

The method preferably further comprises a step of selecting the at leastone standard fastener from a plurality of standard fasteners havingidentical diameters and different lengths in order to provide a selectedbalance weight added to the plate.

An apparatus of the present invention is simply configured and hasdouble functions. The plate is used as a balancing device to selectivelycarry balancing weights while being used as a mounting plate forsupporting the nose cone in its operative position.

Another advantage of the present invention lies in that standardfasteners rather than specially configured balancing weights are used tobalance the shaft of the aircraft engine, resulting in reducedmanufacturing expenses of the aircraft engine. This advantage is evenmore significant when the present invention is applied to small aircraftturbine engines.

Other features and advantages of the present invention will be betterunderstood with reference to a preferred embodiment describedhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings showing by wayof illustration, a preferred embodiment in which:

FIG. 1 is a partial cross-sectional view of an aircraft turbine engine,showing a nose cone mounting plate used as a balancing plate accordingto one embodiment of the present invention;

FIG. 2 is a front elevational view of the mounting plate for the nosecone used in the embodiment of FIG. 1;

FIG. 3 is a partial cross-sectional view of the mounting plate of FIG.2, taken along line A-A, showing the structural details thereof;

FIG. 4 is a rear elevational view of a clinch nut to be attached to therear side of the mounting plate FIG. 3, showing the cut-away forrestraining rotation of the nut;

FIG. 5 is a front elevational view of the nose cone of FIG. 1, showingthe mounting recesses thereon;

FIG. 6 is a rear elevational view of a nose cone of FIG. 1, showing theinner structure thereof;

FIG. 7 is an enlarged portion of FIG. 1, showing an optional applicationof adhesive between the selected standard fastener and the balancingapparatus; and

FIG. 8 is a schematic illustration of examples of a variety of standardfasteners having the same diameters and different lengths in order toprovide a selected balance weight to be added to the balancingapparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, particularly FIGS. 1 and 2, an aircraftturbine engine generally indicated by numeral 10 includes a main shaft12 rotatably supported within an engine casing 14 and being driven by aturbine rotor (not shown) of the engine. A fan integrated blade rotor(IBR) assembly 16 is mounted to a forward end of the main shaft 12 to bedriven in rotation together therewith. A nose cone 18 is mounted to theforward end of the main shaft 12 by means of a mounting plate 20 suchthat an annular airflow inlet is defined at the front opening 22 of theengine casing 14, between the engine casing 14 and the fan IBR assembly16 together with the nose cone 18. The nose cone 18 at its rear sidedefines a central cavity 66 therein to accommodate the forward end ofthe main shaft 12 and, the fan retaining nut 52 engaged with the forwardend of the main shaft 12.

Referring to FIGS. 1, 2 and 3, the mounting plate 20 is a round platepreferably made of a turned steel, having a central aperture 24. A frontannular rim 26 is disposed coaxially with respect to the round plate 20and extends axially forwardly from the front side of the plate 20. Thefront annular rim 26 divides the plate 20 into a central portion 28 andan annular portion 30 much thicker than the central portion 28.

A first group of holes 32 are defined in the annular portion 30 of theplate 20, axially extending through the plate 20. The first group ofholes 32 are tapped respectively to form inner threads therein forselectively engaging standard specification defined fasteners 33, suchas Military Standard (MS) type screws, which will be further describedhereinafter. The first group of the holes 32 are disposed in thecircumference direction of the annular portion 30 of the plate 20, andare preferably spaced apart substantially equally one from another. Thefirst group of holes. 32 are used as attachment points for addingbalance weights to the mounting plate 20. Therefore, it is preferable tohave a great number of the holes 32 disposed in a close relationship oneto another in order to allow placement of a selected balance weightelement in an accurate angular direction. However, the number of theholes 32 should be limited in order to not jeopardize a predeterminedstrength and stability of the mounting plate 20 for safely bearing thedynamic airflow loads on the nose cone 18 during engine operation,particularly during flight operation.

A second group of holes 34 are also defined in the annular portion 30 ofthe plate 20, equally and circumferentially spaced apart one fromanother. In this embodiment, three holes 34 spaced apart by 120 degreesextend axially through the plate 20, for receiving mounting bolts 36therethrough. Each of the holes 34 includes a diametrically reducedsection 37 at the opening thereof defined in the rear side 38 of theplate 20. A clinch nut 40 is attached to each hole 34 at the rear side38 of the plate 20 such that the holes 34 with the attached clinch nuts40 form a plurality of mounting points for mounting the nose cone 18 ofFIG. 1 to the mounting plate 20. Each clinch nut 40 has a diametricallyreduced front section 42 with thin front edges to be inserted into ahole 34 through the diametrically reduced section 37. Upon the insertionof the clinch nut 40 into the hole 34, the thin front edge of the frontsection 42 of the clinch nut 40 is forced to radially expand, therebyengaging the front section 42 of the clinch nut 40 in the diametricallyreduced section 37 of the hole 34.

In order to prevent the clinch nut 40 from rotating together with themounting bolt 36 of FIG. 1 while the bolt 36 is being tightened, aportion of the clinch nut 40 at one side thereof is cut-away (asillustrated in FIG. 4) such that the cut away side 44 is configured anddisposed closely with a section of a rear annular rim 46 of the plate20. The annular rim 46 axially and rearwardly extends from the rear side38 of the plate 20 and has a radius preferably smaller than the radialdistance between the hole 34 and the center of the plate 20. The annularrim 46 is preferably disposed coaxially with the plate 20.

The plate 20 further includes a coaxially disposed rear annular rim 48extending axially and rearwardly from the rear side 38 thereof. Thediameter of the annular rim 48 is smaller than the diameter of theannular rim 46. The inner surface of the annular rim 48 is snuggly fiton a flange 50 of the fan IBR assembly 16 as shown in FIG. 1, to centerthe position of the mounting plate 20 when the mounting plate 20 ismounted to the forward end of the main shaft 12 The central aperture 24of the mounting plate 20 has a diameter greater than the diameter of themain shaft 12 such that there is a clearance between the mounting plate20 and the main shaft 12 when the mounting plate is mounted to theforward end of the main shaft 12 and is centered by the annular rim 48surrounding the annular flange 50 of the fan IBR assembly 16.

As illustrated in FIG. 1, the mounting plate 20 when mounted on theforward end of the main shaft 12, is secured thereto by the fanretaining nut 52 which engages threads on the forward end of the mainshaft 12 and is tightened to axially press the mounting plate 20 againstthe radial wall of the annular flange 50 of the fan IBR assembly 16.Thus, the mounting plate 20 is axially restrained between the fanretaining nut 52 and the IBR assembly 16. As a further advantage of thepresent invention, the mounting plate 20 acts as a washer to preventdamaging the fan IBR assembly 16 when torque is applied to the fanretaining nut 52 to tighten same.

Referring to FIGS. 1, 5 and 6, the nose cone 18 generally includes ahollow conical configuration contoured as a smooth convergent extensionof the fan IBR assembly 16 when the nose cone 18 is mounted to themounting plate 20. The nose cone 18 defines, for example, three recesses54 on the front and outer side thereof, and forms a small portion of aradial surface (not indicated) as the bottom of each recess 54. Therecesses 54 are disposed in accordance with the respective holes 34 inthe mounting plate 20 so that a mounting hole 56 axially extendingthrough the bottom of each recess 54 aligns with a correspondingmounting hole 34 in the mounting plate 20. The mounting bolt 36 can beinserted through the aligned mounting hole 56 in the nose cone 18 andthe mounting hole 34 in the mounting plate 20, further into the attachedclinching nut 40, and can be tightened to threadedly engage theclinching nut 40. The recess 54 provides a space for placing a tool totighten the bolt 36 and the radially extending bottom surface of therecess 54 provides a flat base for the head of the mounting bolt 36 toabut.

The nose cone 18 defines a first outer annular radial surface 58 on therearward and inner side thereof for contacting or being in a closerelationship with the fan IBR assembly 16 such that the nose cone 18covers the front end of the fan IBR assembly 16, thereby forming asmooth annular inner wall of the airflow inlet at the opening 22 of theengine casing 14. The nose cone 18 at the rearward end inner sidethereof further includes a second annular radial surface 60 and anannular rim 62. A radial top surface of the annular rim 62 is preferablydisposed in a radial plane determined by the second annular radialsurface 60 such that both the radial top surface of the annular rim 62and the second annular radial surface 60 abut the forward side of themounting plate 20 at the outer annular portion 30 when the nose cone 18is mounted to the mounting plate 20. The annular rim 62 includes aninner diameter corresponding to the outer diameter of the annular rim 26of the mounting plate 20, and the annular rim 62 of the nose cone 18 issnuggly fitted around the rim 26 of the mounting plate 20 to center thenose cone 18 with respect to the main shaft 12.

An annular recess 64 is defined between the second annular radialsurface 60 and the annular rim 62 with the mounting holes 56 extendingaxially through the bottom of the annular recess 64. The annular recess64 is configured to correspond with the position of the holes 32 of themounting plate 20 (see FIG. 2), and to accommodate one or more of thestandard fasteners 33 which are selectively engaged in one or more ofthe tapped holes 32 for a rotational balance adjustment of the mainshaft 12. The annular recess 64 is further preferably dimensioned torestrain axial movement of the fasteners 33 when the fasteners 33 areengaged in the tapped holes 32 of the mounting plate 20.

During a balance adjustment of the main shaft 12, the mounting bolts 36are removed and the nose cone 18 is dismounted from the mounting plate20. The engine 10 is then started for observation of any rotationalimbalance of the main shaft 12 of the engine, which is well known in theart and will not be further described in detail. When the magnitude ofthe imbalance and the angular direction of imbalance of the main shaft12 are observed, one or more of the tapped holes 32 are determined asbalance weight attachment points and the amount of weight to be added isalso determined.

The next step is to select one or more standard fasteners 33 to act asthe balance weights for engaging in the determined one or more tappedholes 32. The standard fasteners 33 preferably have identical diametersand threads but different lengths such as lengths L1, L2, and L3, asshown in FIG. 8. Selection of appropriate lengths of the standardfasteners 33 will provide a match of the amount of balance weights whichhas been determined.

A further step of shaft balance adjustment is to access the mountingplate 20 through the front opening 22 of the engine casing 14 forinstalling and affixing the selected one or more standard fasteners 33in the determined one or more tapped holes 32. An annular cavity (notindicated) defined within the fan IBR assembly 16 is disposed behind themounting plate 20 in order to accommodate a rear section of the selectedone or more standard fasteners 33, which extends out from the rear sideof the mounting plate 20, regardless of the selected length of thestandard fasteners 33. The selected one or more standard fasteners 33are engaged in the determined one or more tapped holes 32 by means ofthreads. Nevertheless, it is preferred to apply adhesive 68 (see FIG. 7)to the selected one or more standard fasteners 33 and/or the determinedone or more tapped holes 32 in order to provide additional retention ofthe fasteners 33 in the holes 32.

When the main shaft 12 is balanced, the nose cone 18 is placed back inposition to cover the mounting plate 20 and the entire front end of thefan IBR assembly 16, and is then secured to the mounting plate 20 by,for example, the three mounting bolts 36. Each of the mounting bolts 36extends axially through the aligned mounting holes 56 in the nose cone18 and the mounting hole 34 in the mounting plate 20, and is engagedwith the clinch nut 40 by means of threads when the mounting bolt 36 istightened. The bottom of the annular recess 64 is in direct contact orin a close relationship with the heads of the respective mountingfasteners 33 to provide further additional retention of the fasteners33.

Modifications and improvements to the above-described embodiment of thepresent invention may become apparent to those skilled in the art. Theforegoing description is intended to be exemplary rather than limiting.For example, the mounting bolts 36 can be engaged within the mountingholes 34 of the mounting plate 20 by threads defined directly in theholes rather than the clinch nut 40 attached thereto. The clinch nut 40attached to the hole 34 is restrained to prevent rotation by a sectionof the rim 46, in this embodiment. However, it can be restrained by anyother means well known in the art, such as a pin or other well knownstop member affixed to the mounting plate 20 and contacting the clinchnut 40 in a manner such as to prevent rotation thereof. The scope of thepresent invention is therefore intended to be limited solely by thescope of the appended claims.

1. An apparatus for balancing a shaft of an aircraft engine comprising:a plate defining a first group of holes axially extending therethrough,the plate being co-axially attached to the shaft at a forward end of theshaft; a nose cone mounted to the plate; and at least one standardfastener engaging only the plate, the at least one standard fastenerengaging the plate through at least one of the holes in the first groupto thereby add an asymmetric balancing weight to the plate relative to arotational axis of the shaft.
 2. An apparatus as claimed in claim 1wherein the plate further comprises a mounting system independent of thefirst group of holes for mounting a nose cone to the plate.
 3. Anapparatus as claimed in claim 2 wherein the mounting system comprises asecond group of holes axially extending through the plate for receivingmounting bolts.
 4. An apparatus as claimed in claim 3 wherein the platecomprises a plurality of clinch nuts each attached to respective holesof the second group on a rear surface of the plate for engaging therespective mounting bolts.
 5. An apparatus as claimed in claim 4 whereinthe plate comprises means on its rear surface for restraining rotationof the respective clinch nuts.
 6. An apparatus as claimed in claim 2wherein the plate comprises a position element on a forward surfacethereof for co-axially aligning the nose cone with the shaft.
 7. Anapparatus as claimed in claim 1 wherein the plate comprises a positionelement on a rear surface thereof for co-axially aligning the plate withthe shaft.
 8. An apparatus as claimed in claim 7 wherein the platecomprises a central aperture for receiving the shaft extending throughthe aperture with clearance between the shaft and the aperture, theplate being affixed to the shaft by a fan retaining nut secured to theforward end of the shaft, the plate being axially restrained between thefan retaining nut and a radial wall of a fan rotor of the aircraftengine, and the position element on the rear surface of the plate forco-axially aligning the plate with the shaft contacting an axial surfaceof the fan rotor.
 9. An apparatus as claimed in claim 1 wherein at leastone standard fastener is selected from a variety of standard screwshaving identical diameters but different lengths such that one of thescrews with a selected length can be engaged in the at least one of theholes of the first group as the selected balance weight added to theplate.
 10. The apparatus as claimed in claim 1 wherein the nose conedefines a groove configured to accommodate a section of the at least onefastener projecting from the plate.
 11. An apparatus for an aircraftengine comprising: a nose cone of the aircraft engine; at least onebalance weight element; a member centrally mounted to a forward end of arotatable shaft of the aircraft engine, the member including a mountingapparatus mounting the nose cone to the member and the member furtherincluding a balancing apparatus distinct from the mounting apparatus,the balancing apparatus of the member retaining the at least one balanceweight element to the member independently of the nose cone while thenose cone is mounted to the member.
 12. An apparatus as claimed in claim11 wherein the at least one balance weight element is attached only tothe member, from a forward side of the member.
 13. An apparatus asclaimed in claim 11 wherein the nose cone and the mounting apparatus areconfigured to permit the nose cone to be mounted to the member from aforward side of the member and cover the at least one balance weightelement.
 14. An apparatus as claimed in claim 11 wherein the membercomprises a first positioning element to align the member with the shaftfor the central mounting of the member to the shaft.
 15. An apparatus asclaimed in claim 11 wherein the member comprises a second positioningelement to co-axially align the nose cone with the member.
 16. A methodof balancing a shaft of an aircraft engine, the shaft including amounting plate for mounting a nose cone to one side of the mountingplate, the method comprising steps of: with the nose cone unmounted,observing a rotational imbalance of the shaft; and accessing themounting plate through a front opening of a casing of the aircraftengine to install and affix at least one standard fastener in one of aplurality of axial holes of the mounting plate determined during theobservation step to thereby rotationally balance the shaft.
 17. A methodas claimed in claim 16 comprising a step of selecting the at least onestandard fastener from a plurality of standard fasteners havingidentical diameters and different lengths to provide a selected balanceweight added to the plate.
 18. A method as claimed in claim 17 furthercomprising a step of applying adhesive between the selected at least onestandard fastener and the plate.
 19. A method as claimed in claim 17further comprising a step of mounting the nose cone to the mountingplate after the shaft is rotationally balanced, a wall of the nose coneproviding additional retention to the fastener received in the hole. 20.The apparatus as claimed in claim 11 wherein the nose cone defines agroove configured to accommodate a section of the at least one balancingweight element projecting from the member.